In stringed musical instruments, sound is produced from vibrating strings. In an electric guitar, for example, strings are attached on one end to a solid guitar body and on the other end to tuning pegs on a headstock. In order to generate sound, the strings are strummed, with the resultant vibrations generating sound waves.
Unpowered electric guitars generate low levels of acoustic sound on their own. Instead, they are designed to operate with an electric amplification device. When the strings of an electric guitar are plucked, the vibration of the strings generates signals in the form of small electric currents in the magnetic pickups. These signals are fed to an amplifier, which transforms and amplifies the signals into audible sound.
Unlike acoustic guitars (which have hollow bodies), electric guitars typically have solid, heavy bodies. As a result, string vibrations in an electric guitar do not cause much resonance in the body. However, string vibrations in an electric guitar do cause significant resonance in the neck, which is typically much thinner and lighter than the body. The headstock (located at the end of the neck) is subject to the greatest degree of resonance.
Acoustic guitars, because of their construction, are loud enough on their own and do not require any external amplification for practicing or for performing in small rooms. In fact, acoustic guitars may be too loud for practicing late at night in an apartment building.
As discussed earlier, unpowered electric guitars produce acoustic sounds at levels that are too low to be enjoyed without electrical amplification. A typical amplification system involves an amplifier and one or more speakers. They also require electrical power and cabling. This makes spontaneous playing or practicing impractical. In addition, the relatively loud level of sound produced by an amplified electric guitar may be uncomfortable for people in the vicinity. Miniature amplifiers with smaller speakers and/or headphones are known. However, the sound quality of these small speakers is typically inferior (due to their small size). The use of headphones for extended periods of time may be tiring and requires the use of cabling.
U.S. Pat. No. 1,431,773 (Bond) discloses a custom stringed instrument where bridge vibrations are transferred to a diaphragm, with the resultant sound waves projected through a horn. However, Bond requires a custom instrument and cannot be used with conventional stringed instruments.
U.S. Pat. No. 1,762,617 (Dopyera) discloses another custom stringed instrument where bridge vibrations are transferred via mechanical arms to a number of metallic resonators built into the body of the instrument. As with Bond, Dopyera is a custom instrument that cannot be used with conventional stringed instruments. Furthermore, Dopyera produces a distinct metallic sound that is markedly different from those of a wooden acoustic or electric guitar.
U.S. Pat. No. 2,558,893 (Wolff) discloses a custom stringed instrument where a conic speaker is attached directly under the bridge. The bridge is mechanically linked with the apex of the speaker, and the strings' vibrations are transferred to the speaker for amplification. As with Bond and Dopyera, Wolff requires a custom instrument and cannot be used with conventional stringed instruments.
U.S. Pat. No. 4,428,268 (Ingoglia) discloses a self-contained guitar amplification system. The system does not require electricity but instead uses air tubes for transmitting sound from a pickup to an insulated headset. The sound quality of such a system is relatively poor and limited by the size of the air tubes. In particular, small-diameter tubes will significantly restrict sound frequency. Furthermore, the headset in Ingoglia is bulky and tiring to use for prolonged periods.